132 NATURAL HISTORY OF WASPS. 
Fig. 7.—Abdominal spiracle of hornet, seen from the outside, through 
vn the integuments. The diagram shews the crescentic 
opening of the stigma, the tongue extending from 
the convex margin nearly across its orifice, the 
trachea lying parallel with the integuments just 
before it enters the spiracle, and the rudimentary 
hook appearing through its walls. 
round into a hook, the use of which is more obvious, 
as we shall see, in the large thoracic than in the 
abdominal spiracles. The whole organ, as thus dis- 
played, seems to be an oblique valvular mouth-piece 
applied to the end of the trachea, one side of the 
trachea being continuous with the wall of the pouch, 
and the other with the membranous tongue, as shown 
in the accompanying diagram. 
Fig. 8.—Imaginary longitudinal section of an abdominal 
spiracle. 
a, Trachea. 
b, Membranous tongue, closing the orifice of the stigma. 
c, Line of union of the trachea with the spiracle. 
d, Pouch of the spiracle. 
It will be of interest to compare the structure of 
these spiracles with that of the corresponding organs 
of the humble-bee. Unlike the wasp, the humble-bee 
has her chief vocal organs in the abdomen, not in the 
thorax. These have been made the subject of minute 
examination by Dr. H. Landois,* from whose admira- 
ble Monograph on the Vocal Organs of Insects I 
have condensed their description. Here there is a 
horny ring, across which a membrane is spread like 
the head of a drum. This membrane is divided into 
two unequal segments. One, the larger, is rough, 
and strong, and thick, quite up to its edge. The 
other, the smaller segment, is thin and delicate, and 
* * Die Ton-und Stimmapparate der Insecten.’ 8vo. Leipzig, 1867. 
Zeitschrift fur Wissensch. Zoologie, Band XVII, Pl. II, fg. 19. 
